Skimmer apparatus for fluxing light metals



Jan. 24, 1961 w. J. BERGMANN SKIMMER APPARATUS FOR FLUXING LIGHT METALS Filed Jan. 16, 1959 ATTORNEY United States Patent SKIMMER APPARATUS FOR FLUXING LIGHT METALS Werner J. Bergmann, Lower Burrell Township, Westmoreland County, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 16, 1959, Ser. No. 787,221

6 Claims. (Cl. 2283) This invention relates to apparatus for fiuxing light metals, and more particularly to a novel skimmer design for fluxing light metals during casting thereof.

The term light metal, as used herein, refers to aluminum, magnesium, and to alloys thereof wherein these metals predominate.

. In the melting of light metals and transfer of the metal from one receptacle to another, gas is frequently entrapped or dissolved within the molten metal, primarily by reason of the reaction of the metal with even small amounts of moisture in the surrounding environment. This reaction yields free hydrogen, much of which dissolves in the molten metal. Upon casting of the gassed metal, a large proportion of the gas remains in the solidified metal both in dissolved and molecular forms.

It has been a common practice to flux molten light metals in the holding or melting furnaces with chlorine, nitrogen or inert gases to remove any entrapped or dissolved hydrogen. Such processes are time-consuming and the metal is prone to re-absorb gas during any subsequent transfer of the metal.

Skimmers or strainers have been employed in transfer and casting operations to prevent the pass"ge of dross into the mold and entrapment in the casting. Devices have also been designed for use in a mold which com bine skimming with the distribution of the incoming metal below the surface of the pool of molten metal in the mold, thereby catching any floating dross particles and preventing undesirable turbulence. Generaly, such a device consists of a receptacle having discharge ports in the lower portion which also distribute the metal below the surface of the pool of metal in the mold (and in the skimmer receptacle of course).

It is an object of the present invention to provide apparatus for degassing molten light metal in addition to sequestering any dross particles during introduction of molten metal to a mold or other receptacle.

It is a specific object of this invention to provide apparatus for degassing molten light metal while separating dross and distributing the metal within a mold or other receptacle.

Other objects and advantages will be evident from the following detailed specification and the attached drawings, wherein:

Fig. 1 is a sectional view of a skimmer apparatus constructed in accordance with the present invention and in association with an open end mold for the casting of rectangular ingots by the direct chill continuous casting process.

Fig. 2 is a top section of the apparatus along the line 2--2 of Fig. 1.

Fig. 3 is a sectional view of alternative apparatus of circular cross-section as disposed in the casting mold.

It has now been found that molten light metals may be substantially degassed during the transfer from one receptacle to another as in casting by passing them through a skimmer apparatus situated in a mold or other container consisting of an enclosed receptacle having a centrally disposed opening in the top plate for a downspout to introduce the molten metal and ports in the peripheral wall of the receptacle for discharge of the molten metal below the surface of the molten metal in the mold, an upstanding weir spaced from the downspout opening and from the peripheral wall and means for introducing a fiuxing gas into the metal in the bottom of the receptacle between the downspout opening and the upstanding weir.

During operation of the apparatus, metal is introduced through the downspout and fiuxing gas flows upwardly through the metal during its passage through the skimmer chamber between the downspout and the upstanding weir. The metal then flows through the quiescent zone between the weir and peripheral wall, and is discharged through the ports into the mold below the surface of the molten metal therein. The fiuxing gas vents off the metal as it flows through the fluxing and quiescent zones, but especial'y as it flows over an upstanding Weir because of the small increments of metal exposed to the atmosphere.

The means for introducing the fluxing gas into the treating chamber is conveniently provided by a plenum chamber either attached to the bottom plate of the skimmer recept-cle or formed in the bottom of the receptacle as an integral part thereof, although porous or perforated tubes may also be used. The gas-pervious partitioning plate between the plenum Chamber and skimmer may be constructed of a suitable metal or refractory and may be either perforated or porous.

Of course, more than one upstanding weir may be employed, but the space between the outermost weir and the peripheral wall having the discharge ports should be free from inert gas introduction, i.e. quiescent. Also, one or more downwardly extending weirs may be provided to induce tortuous flow of the metal through the apparatus.

It is also necessary to provide a vent for the fiuxing gas in the top plate or cover of the skimmer chamber. Although this may be done by an oversize opening for the downspout it is preferable to provide several vents spaced in the top plate, this being particularly necessary where downwardly extending weirs are utilized and pressure zones would develop.

The tluxing gas may be any of those conventionally empoyed. chlorine, nitrogen, the inert gases, or mixtures thereof. However, the efficiency of the apparatus is such that inert gases are preferred to present corrosion and formation of nitrides or chlorides with the light metals or their alloying, constituents. Generally, argon or helium may be used.

The metal to gas flow rate may vary with the capacity of the apparatus and the efiiciency of treatment desired. Generally, 10 to pounds of metal per cubic foot of fiuxing gas is the flow rate. with a ratio of 30 to 60 pounds per cubic foot being preferred.

In Figs. 1 and 2, suitable apparatus is diagrammatically shown for t e practice of the present invention in the continuous direct chill c sting of rectangular ingots. A skimmer 2 is carried by the supports 4 which rest on the top of the mold w ll 6. The skimmer receptacle 2 is comprised of a vertical peripheral wall 8, a floor plate 9, a plenum chamber 10 below the floor plate 9, and a top cover 12. Several weirs 14 are afiixed to both the floor plate 9 and top cover 12 to induce tortuous flow of the metal being discharged into the skimmer receptacle by the downspout 16 of the casting apparatus. Fluxing gas is supplied to the plenum chamber 10 by the fiuxing gas tubes 18 and passes upwardly through the perforations 20 into the molten metal as it fiows from the downspout 16 to the outermost upstanding weir, but the space between the outermost weir and the discharge ports 22 is free from gas introduction, i.e. quiescent. The lateral ports 22 are adjacent the floor and in "operation are below the surface of the molten metal of the embryo ingot and, of course, the metal level in the skimmer chamber which is represented by the broken lines A. V'ents23 are provided in the cover 12 to permit discharge of fiuxing'gas. As is well known, the skimmer 2 should be spaced from the mold walls 6 so that it does not interfere with the freezing of the molten metal.

In Fig. 3 applratus especially suitable for casting round or square ingots is illustrated, the cylindrical skimmer 50 being carried by the supports 52 which rest on the mold wall 54. The interior of the skimmer receptacle 50 is partitioned by the porous refractory plate 60, which is mounted on the annular support ring 61, to provide a skimmer chamber 56 and a plenum chamber :58 belo'w said plate. The upstanding weir 62 may be afiixed to'the 'partitioning'plate60 or gravitationally rest in place. The downspout 64 and fluxing gas tube '66 extend through the top cover 68. Molten metal discharged from the downspout 64 flows through the chamher in contact with fluxing gas passing upwardly through the porous plate 60 and over the weir 62 before it is discharged through the ports 70 in the peripheral wall 71. The space between the weir 62 and the peripheral wall 71 is free from gas introduction by reason of the impervious support 61. Vents 72 may be provided in the cover 68 for escape of fluxing gas from the skimmer chamber.

Indicative of the efficacy of the present invention are the following examples wherein light metal was tre=ted in apparatus substantially similar to that of Fig. 1 in a continuous casting process employing a large pool of molten metal in the mold.

Example I "An alloy nominally composed of aluminum, 5.6 percent zinc, 2.5 percent magnesium, 1.6 percent copper and 0.3 percent chromium, was cast in two groups of 9 inch diameter ingots, one group being treated according to the present invention at an hourly metal to gas ratio of 39 "pounds per cubic foot of argon fluxing gas, the other utilizing only a conventional skimmer, i.e. one for skimming the dross and distributing the metal only. Tran sverse slices were taken of the ingots and ex mined by the dye penetrantmethod to determine the existence of any porosity, this being considered to be an indication of the presence of any gas voids. The metal treated in the apparatus of the present invention was found to be substantially free from porosity whereas the ingots cast with use of a conventional skimmer showed heavy porosity uniformly distributed throughout the specimen.

Another test of the gas content of the metal was made by taking samples of the molten metal in the trough upstream of the skimmer apparatus of the present invention and of the metal after treatment therein. In this test, the samples of molten metal were frozen under a .partial vacuum of 2-5 mm. and the density determinations made on the resultant castings. The apparatus may be the type described in Light Metals (London), vol. 15, pages 306, 307 (September 1952). The treated metal was frozen under a vacuum of 25 mm., and the castings had' a density of 2.75-2.77 g./cc., where-as specimens from untreated metal have a density of 2.32.4 g./cc.

Example 2 A highly reactive aluminum base alloy containing lithium of the type described in .U.S. Patent 2,381,219 was cast into ingots 6 x 16 inches in cross-sectionutilizing apparatus of the type illustrated in Fig. 1. The hourly metal to gas flow rate was 54 pounds per cubic foot of argon gas. Specimens of the ingots made from treated metal were heated under a vacuum to determine the hydrogen content, as described in Journal of Metals, vol. XIII, p. 1672 (1956), by J. LJBrandt and C.N. Cochran. The hydrogen content of samples of the treated metal was found to be 0.24 ml./ 100 g., whereas hydrogen in untreated specimens of this alloy amounts to 1.52.0

ml./ g., and even higher.

The apparatus of the present invention is particularly adapted to use in special types of continuous casting operations, i.e. the progressive and uninterrupted formation of a cast body in an open ended mold or die positioned in the floor or base of a container and wherein a molten metal pool is maintained within the container completely covering the top of the mold or die. However, it is also applicable to conventional direct chill continuous casting practice so long as the skimmer' is spaced from the mold walls sufficiently to prevent interference with the solidification of the metal to form the ingot. It is obvious to those skilled in the art that the apparatus may also be employed at an intermediate stage in the casting operation or it may be used to treat metal discharged into a holding pot or ladle.

Having thus described the invention, I claim:

1. A skimmer appuatus for fluxing molten light metals during transfer into a container comprising a skimmer chamber defined by a peripheral wall, floor plateand cover, said peripheral wall having ports in the lower portion thereof spaced from the point of introduction of the metal for discharge, of metal after tre"tment into the container below the surface of the molten metal in the container during operation of the apparatus, said cover having a centrally disposed opening therein for introduction of molten metal; at least one upstanding weir in s id chamber spaced between said opening and the said ports in said peripheral wall; means for introducing'a fluxing gas into the bottom of said chamber between said opening and said weir through said floor plate;a'vent in said top plate for discharge of fluxing gas during operation of the apparatus; and means forsupporting said skimmer apparatus in the container during operation spout of a metal transfer system; a plenum chamber disposed below said floor plate; at least one upstanding weir in said skimmer chamber spaced between said opening and the said ports in said peripheral wall, the said floor plate between said opening and said weir being pervious to gas from said plenum chamber and said cover having a vent therein for discharge of fluxing gas from said skimmer chamber; means for introducing fluxing gas into said plenum chamber; and means for supporting said skimmer apparatus in the container during operation thereof.

3. A skimmer apparatus for fluxing molten light metals during transfer into a container comprising a skimmer chamber defined by a peripheral wall, floor plate and cover, said peripheral wall having ports in the lower portion thereof spaced from the point of introduction of the metal for discharge of metal after treatment into the container below the surface of the molten metal therein during operation of the apparatus and said cover-having a centrally disposed opening for receiving the downspout of a metal transfer system; a plenum chamber disposed below said floor plate; at least one upstanding weir in said skimmer chamber spaced between said opening and the said ports in said peripheral wall, the floor plate b'etween said opening and said weir being pervious to gas from said plenum chamber and said cover having vents therein for discharge of gas'from said skimmer chamber; 'atleast one downwardly extending weir aflixed tosaid cover and spaced between said upstanding weir and said peripheral wall; means for introducing fluxing gas into said plenum chamber; and means for supporting said skimmer apparatus in the container during operation thereof.

4. A skimmer apparatus for fiuxing molten light metals prior to casting thereof comprising a receptacle defined by a peripheral wall, base and cover, said cover having a centrally disposed opening therein for introduction of molten metal; a horizontally disposed floor plate partitioning the receptacle into a skimmer chamber and into a plenum chamber in the bottom thereof, said peripheral wall having ponts adjacent the bottom of said skimmer chamber spaced from the point of introduction of the metal for discharge of metal after treatment into the mold below the surface of molten metal therein during operation of the apparatus; at least one upstanding weir affixed to said partitioning plate spaced between said opening and the said ports in said peripheral Wall, said partitioning plate between said opening and said Weir being pervious to gas from the plenum chamber; a vent in said cover for discharge of fluxing gas during operation of the apparatus; means for supplying fluxing gas to said plenum chamber; and means for supporting said receptacle in the mold during operation thereof.

5. A skimmer for fiuxing molten light metals prior to the casting thereof comprising a receptacle defined by a cylindrical peripheral Wall, base and cover, said cover having a centrally disposed opening for receiving the downspout of a metal transfer system; a horizontally disposed plate partitioning said receptacle into a skimmer chamber and into a plenum chamber in the bottom thereof, said peripheral wall having ports adjacent the bottom of said skimmer chamber for discharge of metal into the mold below the surface of molten metal therein during operation of the apparatus; an upstanding annular Weir affixed to said partitioning plate and spaced from said opening and from said ports in said peripheral wall, the plate between said opening and said Weir being pervious to gas from said plenum chamber and said cover having vents therein for discharge of gas from said skimmer chamber; means for introducing fiuxing gas into said plenum chamber; and means for supporting said receptacle in the mold during operation thereof,

6. Apparatus in accordance with ciaim 5 wherein a downwardly extending annular weir is afiixed to the said cover and spaced between the said upstanding weir and said peripheral Wall.

References Cited in the file of this patent UNITED STATES PATENTS 1,323,583 Earnshaw Dec. 2, 1919 1,808,370 Munson June 2, 1931 1,983,578 Chandler Dec. 11, 1934 2,135,183 Junghans Nov. 1, 1938 2,224,303 Junghans Dec. 10, 1940 2,821,472 Peterson et al. Jan. 28, 1958 

